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Baby star illuminates icy wall between rocky planets and giants

The water snow line of V883 Orionis’s protoplanetary disc is shown in orange

Cieza,L.A.etal.Nature

By Ken Croswell

Sometimes you catch a lucky star. The flare-up of a star near the Orion Nebula has revealed for the first time the icy boundary between small rocky worlds and their giant neighbours in a planet-making disc.

New planets form out of a disc that spins like a pizza around a baby star. Close to the star, the discs are so hot that only rock and iron condense, which explains why Earth and its neighbours are small and rocky. But beyond the so-called “water snow line”, water freezes and joins rock and iron to build the cores of giant planets.

Unfortunately, this snow line normally lies a little farther from a young star than Earth is from the sun, making it impossible to see directly. So nobody had ever spotted one – until now.

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Lucas Cieza at Diego Portales University in Santiago, Chile, and colleagues used the ALMA array of radio telescopes in Chile to observe a disc around the young star V883 Orionis, which is 1350 light years from Earth.

Snow line spotted

They hoped to see signs of newborn planets. Instead, they saw a ring around the star that was just the right temperature – 105 kelvin – for a snow line.

“It is the first detection of a water snow line,” Cieza says.

The team owe their success to the star itself, which erupted sometime before 1889. It resembles another stellar youngster, a star named FU Orionis, which could only be seen through a telescope until late 1936, when it suddenly shone at least 100 times as brightly – enough to observe through binoculars. It’s still shining brightly today.

The new light from V883 Orionis’s similar eruption vaporised so much water ice that the snow line is now 42 times farther out than Earth is from the sun. That made it easier to detect.

Because water ice is critical to the birth of giant planets, Cieza says these FU Orionis-like outbursts could affect how planets form. Most young stars probably experience such outbursts, so Cieza says future simulations of developing solar systems should include their effects.